Complain at fish (4)
I believe the answer is:
carp
I believe this clue is a double definition.
'complain' is the first definition.
(thesaurus)
'fish' is the second definition.
(carp is a kind of fish)
'at' acts as a link.
(Other definitions for carp that I've seen before include "Swimmer" , "Keep complaining" , "Fish < and beef" , "Raise petty objections about goldfish" , "Raise trivial objections" .)